1. Field of the invention
The present invention relates to a frequency-offset removal method and apparatus in which frequency offset is reliably removed even if a burst signal is received.
2. Description of Background
Generally, in radio communication, when high frequency radio waves are received by a receiver in which the received signals are demodulated to base-band signals, an oscillating frequency provided from a local oscillator in the receiver must be adjusted to a specific frequency which is identical with the carrier frequency of the received signals.
Therefore, the frequency of the local oscillator is adjusted by utilizing a phase lock loop (PLL) circuit or an auto frecuency control (AFC) circuit so that the frequency is synchronized with the received signals in a conventional receiver. In detail, the difference between the frequency from the oscillator and the carrier frequency of the received signals is detected as phase difference (in case of PLL), then the oscillating frequency of the local oscillator is controlled corresponding to the level of the phase difference so that the local oscillator can provide the oscillating signal identical with the carrier frequency of the received signal.
The above conventional method can be applied to digital communication.
However, when the received signal is formed from a burst signal occuring in the communication of the time division multiple access (TDMA), the burst signal is too short to allow adjustment of the frequency of the oscillator. Therefore, it is difficult for the conventional oscillator to adjust the oscillating frequency by utilizing the PLL circuit. In particular, when the received signal is distorted by multipath fading in the communication with a mobile station, it is substantially impossible to adjust the oscillating frequency by using the PLL circuit. Also, to synchronize the above signals in an extremely short period, which is required when a hand-off occurs, is impossible in the mobile communication.
As mentioned above, in the conventional method for adjusting the oscillating frequency by utilizing the PLL circuit, when one burst training signal or preamble signal is too short in such digital mobile communication utilizing the TDMA method, the removal of the frequency offset is very difficult. In particular, when the hand-off operation is carried out in the TDMA mobile communication method, the synchronous adjustment is required at a high speed. Therefore, a frequency-offset removal apparatus which can be applied in the TDMA mobile communication method is required.